Bump pygments from 2.1.3 to 2.7.4

Bumps [pygments](https://github.com/pygments/pygments) from 2.1.3 to 2.7.4.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.1.3...2.7.4)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

dev-requirements.txt

@@ -50,7 +50,7 @@ py==1.4.31
     #   tox
 pyflakes==1.0.0
     # via flake8
-pygments==2.1.3
+pygments==2.7.4
     # via sphinx
 pytest==2.9.1
     # via -r dev-requirements.in